I. Field of the Invention
The present invention relates generally to motorized concrete finishing trowels. More particularly, the present invention relates to motor powered riding trowels of the type classified in United States Patent Class 404, Subclass 112.
II. Description of the Prior Art
It has long been recognized by those skilled in the art that freshly placed concrete must be appropriately finished. Proper and timely finishing insures that desired surface characteristics including appropriate smoothness and flatness are achieved. Motorized riding trowels are ideal for finishing large very areas of plastic concrete quickly and efficiently, and such trowels are rapidly becoming the industry standard.
A typical power riding trowel comprises two or more bladed rotors that project downwardly and frictionally contact the concrete surface for finishing. These rotors are driven by one or more motors mounted on the frame. The motors drive suitable reduction gearboxes (i.e., 20:1 reduction) to revolve the rotors. The riding trowel operator sits on top of the frame and controls trowel movement with a steering system that tilts the axis of rotation of the rotors. The weight of the trowel and the operator is transmitted frictionally to the concrete by the revolving blades. The unbalanced frictional forces caused by rotor tilting enable the trowel to be steered.
Holz, in U.S. Pat. No. 4,046,484 shows a pioneer, twin rotor, self propelled riding trowel. U.S. Pat. No. 3,936,212, also issued to Holz, shows a three rotor riding trowel powered by a single motor. Although the designs depicted in the latter two Holz patents were pioneers in the riding trowel arts, the devices were difficult to steer and control.
Prior U.S. Pat. No. 5,108,220 owned by Allen Engineering Corporation, the same assignee as in this case, relates to an improved, fast steering system for riding trowels. Its steering system enhances riding trowel maneuverability and control. The latter fast steering riding trowel is also the subject of U.S. Pat. No. Des. 323,510 owned by Allen Engineering Corporation.
Allen Engineering Corporation U.S. Pat. No. 5,613,801 issued Mar. 25, 1997 discloses a power riding trowel equipped with twin motors. The latter design employs a separate motor to power each rotor. Steering is accomplished with structure similar to that depicted in U.S. Pat. No. 5,108,220 previously discussed.
Allen U.S. Pat. No. 5,480,257 depicts a twin engine powered riding trowel whose guard structure is equipped with an obstruction clearance system. When troweling areas characterized by projecting hazards such as pipes or ducts, or when it is necessary to trowel hard-to-reach areas adjacent walls or the like, the guard clearance structure may be retracted to apply the blades closer to the target region.
Allen U.S. Pat. No. 5,685,667 depicts a twin engine riding trowel using "contra rotation." For enhanced stability and steering, the rotors rotate in a direction opposite from that normally expected in the art.
As freshly poured concrete "sets," it soon becomes hard enough to support the weight of the specialized finishing trowel, so pan finishing can begin. By starting panning while concrete is still "green," within one to several hours after pouring depending upon the concrete mixture involved, "super-flat" and "super-smooth" floors can be achieved. The advent of more stringent concrete surface finish specifications using "F" numbers to specify flatness (ff) and levelness (fl), dictates the use of pans on a widespread basis.
The panning process comprises three different recognizable stages. In the initial "brake open" stage, the rotors are ideally driven between 40 and 65 RPM. As the concrete hardens, the pan floating stage occurs, involving rotor speeds between 70 and 95 RPM. The last phase of pan floating, the "fizz stage," uses an increased rotor speed of between 95-125 RPM. At present these RPM requirements are achieved simply by varying motor speed.
Pan finishing is normally followed by medium speed blade finishing, after the pans are removed from the rotors. A developing technique is the use of "combo blades" during the intermediate "fuzz stage" as the concrete continues to harden. So-called "combo-blades" are a compromise between pans and normal finishing blades. They present more surface area to the concrete than normal finishing blades, and attack at a less acute angle. The rotors are preferably turned between 100 to 135 RPM at this time. Finishing blades are then used, and they are rotated between 120 to 150 RPM. Finally, the pitch of the blades is changed to a relatively high contact angle, and burnishing begins. This final trowel finishing stage uses rotor speeds of between 135 and 165 RPM.
Modern large, high power riding trowels are noted for their speed, horsepower, and efficiency. Such trowels, however, are not without weaknesses. For example, the drive train and the high power motors are subject to substantial stress during operation. Motor loading varies as the rotor RPM requirements change. The motors function most efficiently at a given operating point in their characteristic horsepower-RPM and torque-RPM curves. Especially with diesel engines, optimum torque and horsepower requirements are achieved over a limited RPM range.
The engines on most riding trowels directly power the reduction drive gear boxes connected to the rotor shafts. The incoming shaft speed of the conventional rotor gear box is the same as the motor RPM. The output shaft speed (i.e., rotor speed) is geared down, approximately 20:1. Engine RPM is usually the key variable elated to output power. But if the engine speed increases too much, excessive power may be developed and the finishing mechanism may rotate too fast. For example, the initial panning stage requires relatively high power because of the viscous character of the still-wet concrete, but relatively low rotor speeds are desired. Since the rotors are driven through a fixed ratio, established by the gearbox and pulleys, optimum engine power often cannot be obtained during panning without risking excessive rotor speeds.
It is thus desirable to provide a riding trowel wherein the engine and gear boxes can operate at ideal speeds over a wide range of finishing conditions. Although it has been suggested by others in the finishing trowel industry (i.e., Bartell Industries) to vary gear box input shaft speeds with a "high-low" or "torque converter" attachment, we are aware of no prior art means for reliably providing a continuously variable gear box drive system that holds a given user-selected gear ratio during the different pan and blade finishing stages discussed above. An appropriate variable ratio drive train must be compatible with existing trowel motors, frames and gear boxes to minimize cost. Further, it must not overly complicate the drive train so that operating efficiency and reliability are preserved.